Jacket for offshore structure

ABSTRACT

A jacket  4  for an offshore structure comprises a plurality of jacket modules  6  stacked one above the other, each of the jacket modules having multiple hollow legs  10;  and a plurality of piles  8  passing through the hollow legs  10  for securing the jacket  4  to the sea-bed; wherein the piles  8  include: foundation piles  8   a  passing through legs  10  of a lowermost jacket module  6   a,  with a lower part of each foundation pile extending downward below the lowermost jacket module  6   a  into the sea-bed, and an upper part of each foundation pile extending upward out of the tops of the legs  10  of the lowermost jacket module  6   a  and into legs  10  of a second jacket module  6   b  located atop the lowermost jacket module  6   a  such that the upper parts of the foundation piles  8   a  extend into the legs  10  of the second jacket module  6   b  by a height less than the height of the jacket modules  6;  and follower piles  8   b  within the legs  10  of the second jacket module  6   b  located atop the upper parts of the foundation piles  8   a  and extending upward out of the tops of the legs  10  of the second jacket module  6   b.

The present invention relates to a jacket for an offshore structure andto a method for installing such a jacket. The offshore structure may forexample be a foundation for a wind turbine.

As a consequence of increased energy demands and a desire to generate alarger amount of so called renewable energy an offshore wind industryhas developed. An offshore wind farm avoids many of the restrictionsplaced on land-based wind farms by planning regulations and suchlike. Italso makes transport and installation of large parts to the wind farmsite considerably easier, since it is not necessary to design componentsto be transportable by road, which can restrict the size and weight ofindividual components. Many designs of offshore wind turbine have beeninstalled and more are in development.

The turbine mechanism and the tower can be derived from land basedtechnologies to some extent, since the basic requirements are the same,although offshore locations will permit windmills of greater size andhence turbines of considerably larger generating capacity. However, thefoundation for an offshore turbine is naturally considerably differenceto land-based foundations. Current plans involve wind farms placed inrelatively deep water, perhaps depths in the range of 20 to 50 metresand with these depths the foundation becomes a significant part of theoverall structure. Moreover, offshore wind farms may require themanufacture and installation of tens or hundreds of wind mills. As aresult there is a need for a structure that can be manufactured andinstalled efficiently.

Viewed from a first aspect, the present invention provides a jacket foran offshore structure, the jacket comprising: a plurality of jacketmodules stacked one above the other, each of the jacket modules havingmultiple hollow legs; and a plurality of piles passing through thehollow legs for securing the jacket to the sea-bed; wherein the pilesinclude: foundation piles passing through legs of a lowermost jacketmodule, with a lower part of each foundation pile extending downwardbelow the lowermost jacket module into the sea-bed, and an upper part ofthe each foundation pile extending upward out of the tops of the legs ofthe lowermost jacket module and into legs of a second jacket modulelocated atop the lowermost jacket module such that the upper parts ofthe foundation piles extend into the legs of the second jacket module bya height less than the height of the jacket modules; and follower pileswithin the legs of the second jacket module located atop the upper partsof the foundation piles and extending upward out of the tops of the legsof the second jacket module.

The invention hence provides a modular construction that enables ajacket to be prefabricated in easily transportable parts. For example,the jacket modules might be between 12 and 18 metres in height, thefollower piles may be the same length as the height of the modules, andthe foundation piles might have lengths of 25 to 45 metres more than theheight of the modules, the extra length including a length that extendsinto the sea-bed. The length of the pile in the sea-bed is determinedbased on soil conditions and on the expected loading on the offshorestructure. These parts can be assembled upon installation without theneed for specialist vessels or complicated moorings. The parts are smallenough and light enough to be conveyed on relatively small barges andcan be lifted by standard cranes. Vessels of an appropriate size toinstall the modular system are more widely available and cheaper tooperate than the larger and more complex vessels that would be requiredin order to handle larger components. The modular system can also beinstalled in a time efficient manner, more quickly more complexinstallations and also with several small vessels operating in parallel.Thus, a greater number of jackets can be installed in a given time.

The piles extend throughout the entire height of the modules but areseparated into different segments with the piles and jacket modulesoverlapping. This provides a strong and stable structure. Preferably thepiles in the legs of a lower jacket module extend upwardly into the legsof the next jacket module up by a distance of 20% to 50% of the heightof the modules.

The upper part and lower part of each foundation piles may be separatepile segments. This has the advantage of reducing the maximum length ofthe foundation pile components. With this arrangement there may be ajoint between the upper part and lower part. Preferably this joint hasthe same coupling as the coupling between the foundation pile andfollower pile, which is discussed in more detail below. In aparticularly preferred arrangement of this type the upper part of thefoundation pile may be similar or identical to the follower piles, andhence may have the same dimensions and shape.

In an alternative arrangement the foundation pile is formed in a singleuninterrupted length with the upper part and lower part being top andbottom parts of this single length.

In a preferred embodiment the piles are arranged to extending upwardlyinto the legs of the next jacket module by different amounts. Inparticular, there may be at least a first guide pile that extendsupwardly into the legs of the next jacket module by a greater lengththan the other piles. This guide pile can be used as a first landingpoint for the next jacket module (or upper structure) when it isinstalled, avoiding the need to align the legs of this module with allfour piles at once. Preferably, there is also a second guide pile thatextends upwardly into the legs of the next jacket module by a lengthless than the first guide pile but greater than the remaining piles.Using two guide piles in this way means that with two steps of landingthe next jacket module first on the first guide pile, and then on thesecond guide pile, the modules can be easily aligned.

The amount of overlap may for example be at least 4 m for a 12 m highjacket module. With this size of module, the first guide pile may extendinto the legs of the next jacket module by 5.5 m, the second guide pilemay extend into the legs of the next jacket module by 4.5 m and theremaining two piles may extend into the legs of the next jacket moduleby 4 m.

The second jacket module may be the uppermost jacket module. However, itis preferred to use at least one additional jacket module and alsoadditional follower piles. Thus, the jacket may comprise: a third jacketmodule atop the second jacket module, with the follower piles in thelegs of the second jacket module being a first set of follower piles andextending upward into the legs of the third jacket module; and a secondset of follower piles within the legs of the third jacket module locatedatop the first set of follower piles and extending upward out of thetops of the legs of the third jacket module. There may be furthermodules and piles, for example a fourth jacket module and a third set offollower piles. In some preferred embodiment the jacket includes a totalof only three jacket modules.

Preferably the jacket comprises an upper structure mounted on theexposed parts of the uppermost follower piles above the uppermost jacketmodule. The upper structure could be any desired structure forcompleting the offshore installation or for providing a platform forinstallation of additional structures or apparatus. In a preferredembodiment, the upper structure is a transition piece for mounting atower, such as a wind turbine tower, on the jacket.

The transition piece is preferably pre-fabricated onshore and may have acentral structure for supporting a tower or the like and outerconnecting parts for coupling to the exposed parts of the uppermostfollower piles. The outer connecting parts may be hollow cylinders withdimensions similar to the hollow legs of the jacket modules. The centralstructure and outer connecting parts may be joined by a platformstructure and/or by beams or similar. In one preferred arrangement thetransition piece is integrated with a tower such as a wind turbinetower.

It is preferred for the piles to be secured within the legs of thejacket modules by grout. The grout may be grout that has been pumpedinto the legs around the piles after assembly of all the jacket modulesand piles is completed. When an upper structure is present the groutpreferably also surrounds the piles within the outer connecting partsand hence secures the upper structure to the jacket. With the use ofgrout the hollow legs should have a cross-section larger than thecross-section of the piles, preferably a cross-section arranged topermit grout to surround the piles. For example the legs may be roundtubes of 1 to 1.5 metres diameter and the piles may be cylinders with adiameter 20% to 40% smaller than the diameter of the legs.

The grout ensures a secure connection between the piles and jacketmodules and provides the necessary tensile strength to preventinstability and failure of the jacket when vertical or horizontal loadson the offshore structure generate tensile loads on the jacket. Othercoupling methods for joining the piles and jacket modules are possible.For example the piles and legs may be joined via mechanical couplingssuch as bolts passing through the legs and piles. However, grout ispreferred. Grouting does not require any accurate vertical alignment ofthe piles and legs. Also, the grouting of each leg of the jacket can becompleted in one simple operation by pumping grout into the base of theleg using conventional grouting techniques.

With the overlap of the piles and the hollow legs of the modules it isnot necessary for the adjacent jacket modules to incorporate anycoupling mechanism for transferring loads, aside from abutment ofadjacent modules to transfer compression loads. However, it is preferredfor the jacket modules to include connectors for providing a seal aboutthe joint between adjacent hollow legs, in order to more effectivelycontain grout within the hollow legs. The connectors may take the formof a connecting ring provided at the base or top of each tubular leg ofthe jacket modules, at least a portion of the ring having a diameterlarger than the outer diameter of the legs in order to overlap with thetop part of a corresponding leg of an adjacent jacket module. Theoverlap is relatively small, for example it may be about a quarter ofthe width/diameter of the legs. The connectors may include a tapered orwedge shaped portion to guide the connectors onto the adjacent jacketmodule and/or to provide a tight friction fit and seal one leg to theother. The connectors may incorporate an O-ring or similar sealingmechanism. It is preferred for the legs and connector parts to begenerally circular although the invention is not limited only tocircular geometries.

Preferably the hollow legs include internal lugs for locating the pilesat the centre of the legs and spacing the piles away from the walls ofthe legs. Advantageously this ensures an even and accurate spacing forgrout injection. It also ensures that adjacent piles are aligned withone another.

As explained above, the piles are placed atop one another inside thehollow legs of the jacket modules. Preferably, the ends of the piles areshaped for interlocking fit with the next pile. Hence, the followerpiles may be provided with a male coupling and one end and a femalecoupling at the other end. Preferably the female coupling is designedfor a friction fit with the male coupling. This ensures a positiveconnection and alignment of the piles, and a good contact surface fortransfer of compression loads. When a grouted system is used, thisavoids the need for welding of the piles to one another as is requiredin some prior art constructions. By avoiding the need for welding thecomplexity of the offshore installation operation can be considerablyreduced. The installation operation can also be completed in less time.The piles can be simply lowered into the hollow legs and there is noneed for offshore welding equipment or complicated support arrangementsto hold two piles in place whilst welding occurs.

The upper parts of the foundation piles may be provided at their upperends with a male or female coupling, as appropriate. The lower ends ofthe lower parts of the foundation piles may have a conventional geometryto facilitate driving the pile into the sea-bed. Preferably the upperends of the upper parts of the foundation piles include a flat surfaceto permit connection of a hammer to drive the pile into the sea-bed.

The follower piles are preferably provided with a coupling at a firstend with a geometry that corresponds to the coupling geometry of the topend of the foundation pile. The opposite end of the follower piles mayhave a coupling of the opposite type. Using similar couplings for thefoundation piles and follower piles means that there are only twocoupling types. The tooling required to manufacture the piles is simpleras a result.

In one preferred embodiment the upper ends of the upper parts of thefoundation piles comprises a shoulder around a protruding conical orfrustoconical element. The follower piles in this embodiment hence areoriented with a female coupling at their lower end to accommodate theprotruding part of the upper ends of the foundation piles. This femalecoupling may comprise a socket with a conical or frustoconical internalvolume. The other end of the follower pile preferably has the same malecoupling geometry as the top end of the upper part of the foundationpile.

The jacket modules may comprise the hollow legs with trusses or beamsbetween the legs. The lowermost jacket module may further comprise mudmats at the bases of the hollow legs.

Preferably the jacket modules are designed with an arrangement of legsthat is rotationally symmetrical about an axis extending parallel withthe legs, i.e. an axis that is generally vertical when the jacket moduleis in its installed orientation. The degree of rotational symmetrypreferably corresponds to the number of legs. For example, if the jacketmodule has three or four legs then it should have a threefold orfourfold rotational symmetry. The jacket modules may be entirelyrotationally symmetrical. For example, the jacket modules maycomprise-symmetrically arranged legs joined by similar trussarrangements.

With a symmetrical arrangement when placing one jacket module atopanother it s only necessary to align the legs and there is no otherrestriction on the relative orientation of the two modules. This canlead to significant benefits when installing the modules. With costefficient small vessels the vessel generally must be anchored in a setorientation relative to the wind and/or wave direction. The moduleswould be loaded onto the vessel in a particular orientation and hencelifted and installed in that orientation. If a symmetrical constructionwas not used then in the event that the wind and/or wave directionchanged during installation it would not be possible to continue theinstallation using small cost effective vessels without complicatedadditional equipment to rotate the modules by a significant amount whenlifted. With a symmetrical construction the same small and efficientvessel can continue the installation even if the prevailing sea and windconditions change, since there would only ever need to be a smallrotation of the jacket modules to align the legs with the previouslyinstalled modules. For example, with a fourfold symmetry the maximumrotation would be ±22.5° as compared with ±180° for a non-symmetricalsystem.

The jacket may include a J-tube for passage of cables and the like fromthe sea-bed to the top of the jacket. Preferably the J-tube includes anangled section at the base of the lowermost jacket module, which may beprefabricated with the lowermost jacket module. The angled section maycomprise a lower generally horizontal part for receiving a cable runninggenerally horizontally along the floor, a bend that turns the cablethrough about a right angle, and an upper generally vertical part sothat the cable exits the angled section running generally vertically.Conventionally, a J-tube will also include a vertical section of tubingrunning from the angled section to the top of the jacket. In preferredembodiments however the J-tube of the present jacket does not includesuch a vertical section. Instead, the cabling is passed from thegenerally vertical exit portion of the angled section to the top of thestructure without being enclosed in a tube. It has been found that it isnot necessary to enclose the cabling for many offshore structures,including wind turbines. Omitting the vertical tubing makes theconstruction and installation of the jacket simpler, and also makes iteasier to install cables through the J-tube. There may be cablerestraints spaced apart up the height of the modular structure, such asrings mounted on legs of the modules. The use of cable restraints keepsthe cables in place during wave and tidal water movements.

Preferably, the J-tube is located adjacent to a leg of the lowermostmodule and may be clamped to the leg. In a preferred embodiment, theJ-tube can be rotated in order to direct the lower generally horizontalpart of the angled section in any desired direction. This means that theJ-tube can receive cabling running from any direction, and hence avoidsthe need to orient the jacket relative to the direction of the cabling.In a particularly preferred embodiment there may be two J-tubes mountedadjacent to two opposite legs of the lowermost jacket module. The twoJ-tubes are preferably both rotatable. This means that the jacket canreceive cabling from any direction without the need to run the cablingbeneath or through the jacket before it reaches a J-tube.

The advantages of symmetry apply to installation of the upper structureas well. Preferably this structure is arranged to be installed in anyorientation relative to the piles extending from the symmetrical legs ofthe uppermost jacket module. This is of particular advantage when theupper structure includes parts that should be oriented in a particularway relative to the prevailing wind direction, for example access wayssuch as gangways and/or boarding platforms that should desirably beplaced leeward of the structure, the leeward direction being to the leeof the jacket relative to usual weather conditions.

Hence, in a particularly preferred embodiment the jacket comprises anupper structure such as a transition piece that is prefabricated with anaccess way, wherein the jacket module has a rotationally symmetricalarrangement and the upper structure is hence able to be oriented asdesired upon installation.

The jacket modules may have three or more hollow legs. In a preferredembodiment four legs are used. This allows the jacket modules to have asquare cross-section and cube shaped modules may be used. With a squareand preferably cubic shape the jacket modules can be formed by a simpletruss structure with similar joints and trusses at each side. This makesmass production of the jacket modules simpler and a square or cubicshape also permits effective use of space when storing and transportingthe modules.

A preferred embodiment is intended for use in water depths of 25 m to 60m, and may have jacket modules of a generally cubic truss constructionwith legs at each corner and a height/width of between 10 m and 20 m. Inpreferred arrangements using three jacket modules the total height ofthe jacket, excluding the upper structure will hence be 30 m to 60 m.The jacket may be designed to extend 10 m to 15 m above water level whenthe foundation is installed on the sea-bed.

The jacket has been developed with a focus on the offshore wind industryand hence in a preferred embodiment the jacket is a foundation for anoffshore wind turbine. The invention extends to an offshore wind turbinestructure incorporating the described jacket. However, the jacket mayalso be advantageously utilised in other offshore sectors such as oiland gas.

Viewed from a second aspect, the invention provides a method ofinstallation of a jacket for an offshore structure, the jacketcomprising: a plurality of jacket modules, each of the jacket moduleshaving multiple hollow legs; and a plurality of piles; wherein themethod comprises: locating a first, lowermost, jacket module on thesea-bed; installing foundation piles through the legs of this lowermostjacket module such that lower parts of the foundation piles extenddownward below the lowermost jacket module into the sea-bed and upperparts of the foundation piles extend upward out of the tops of the legsof the lowermost jacket module by a height less than the height of thejacket modules; locating a second jacket module atop the lowermostjacket module such that the upper parts of the foundation piles extendinto the legs of the second jacket module; and installing follower pileswithin the legs of the second jacket module such that the follower pilesare located atop the upper parts of the foundation piles and extendupward out of the tops of the legs of the second jacket module.

The method of the invention enables installation of the jacket withoutthe need for specialist vessels or complicated moorings. The parts aresmall enough and light enough to be conveyed on relatively small bargesand can be lifted by standard cranes. The jacket modules, piles andother parts may have features as discussed above in relation to thefirst aspect of the invention.

Thus, although the second jacket module may be the uppermost jacketmodule it is preferred to use at least one additional jacket module andalso additional follower piles. The method may hence comprise: thefollower piles in the legs of the second jacket module being a first setof follower piles; locating a third jacket module atop the second jacketmodule such that the first follower piles extend into the legs of thethird jacket module; and installing a second set of follower pileswithin the legs of the third jacket module such that the second followerpiles are located atop the first follower piles and extend upward out ofthe tops of the legs of the third jacket module.

In a preferred method the foundation piles are installed extendingupward from the lowermost jacket module by a length of from 20% to 50%of the height of the jacket modules. The follower piles may protrudeupward from the second jacket module and optional further jacket modulesby a similar length.

In a preferred embodiment the piles are installed such that they extendupwardly into the legs of the next jacket module by different amounts.This may be done by installing the foundation piles at different depthsin the sea-bed, or by providing foundation piles of different lengthsfor installation at the same depth in the sea-bed. When the followerpiles all have the same length, which is preferably a lengthcorresponding to the height of the modules, then the differing heightsof the foundation piles will be recreated above the second jacket moduleby the exposed parts of the follower piles.

There may be at least a first guide pile that extends upwardly into thelegs of the next jacket module by a greater length than the other piles,and the method may include Iodating the next jacket module by firstlanding one leg on the first guide pile, and then aligning the modulewith the other piles. Preferably, the method includes installing asecond guide pile that extends upwardly into the legs of the next jacketmodule by a length less than the first guide pile but greater than theremaining piles. With the use of a second guide pile, the method ofinstalling the next jacket module may include landing a first leg of thenext jacket module on the first guide pile, and then aligning a secondleg of the jacket module with the second guide pile and landing thesecond leg on the second guide pile so that the remaining legs and pilesare aligned.

The method may comprise mounting an upper structure on the exposed partsof the uppermost follower piles above the uppermost jacket module. Theupper structure may be a structure as described above. Prior to mountingthe upper structure the exposed parts of the piles are preferably cut toa desired uniform length.

In a preferred embodiment the method comprises grouting about the pilesinside the hollow legs in order to thereby secure the piles within thelegs. The grout is preferably pumped into the legs around the pilesafter assembly of all the jacket modules and piles is completed. When anupper structure is present grouting preferably also occurs aftermounting of the upper structure so that the grout surrounds the pileswithin outer connecting parts of the upper structure and hence securesthe upper structure to the jacket. The grouting of the upper structureto the piles preferably occurs in a separate step to the grouting of thejacket module to the piles, with the step of grouting the jacket moduleto the piles being completed before the upper structure is fitted.

The method may include grouting in one of the following sequences:

1. Grout the legs of the jacket modules and the connection to the upperstructure in one sequence, then install the tower or other functionalpart atop the upper structure.

2. Grout all jacket modules in a first step, with the connections to theupper structure being grouted in a second, separate step, afterinstallation.

3. Grout a few metres (for example, 4 metres) at the base of thelowermost jacket module, plug, then grout to the top, with the secondgrouting stage optionally including grouting of the upper structure tothe jacket.

The step of locating a jacket module atop a lower jacket module mayinclude aligning the legs of the jacket module. Preferable the legsinclude connectors as discussed above and the alignment uses tapered orwedge shaped parts of the connectors to guide the connectors onto theends of the legs of the adjacent jacket module.

In preferred embodiments the piles have male and female couplings asexplained above and the method comprises locating the male part of onepile within the female part of another pile during installation of thefollower piles, as the follower pile is lowered into the hollow leg.

The preferred jacket modules are symmetrical as described above. Themethod of the invention may comprise aligning the legs of the jacketmodules when placing one jacket module atop another. Preferably themethod includes installing an upper structure comprising outerconnecting parts and the method comprises aligning the outer connectingparts with the exposed parts of the uppermost follower piles. Inpreferred embodiments the upper structure is prefabricated with anaccess way and during installation of the upper structure it is alignedwith the access way leeward of the jacket.

The method may be used as a part of a method of installation of anoffshore wind turbine and hence the method may comprise installing anupper structure in the form of a transition piece for holding a turbinetower and installing a tower, turbine and wind mill on the jacket. Themethod may comprise connecting cabling to the jacket, preferably via aJ-tube arrangement as described above. In the preferred embodiment wherethe J-tube is rotatable the method may comprise rotating the J-tube todirect it toward the direction of the cabling on the sea-bed.

It will be appreciated that the jacket structure is essentiallymanufactured and transported in kit form. Hence, viewed from a thirdaspect, the invention provides a kit of parts for a jacket, the kit ofparts comprising: a plurality of jacket modules arranged to be stackedone above the other, each of the jacket modules having multiple hollowlegs; and a plurality of piles arranged for passing through the hollowlegs for securing the jacket to the sea-bed; wherein the piles include:foundation piles for passing through legs of a lowermost jacket module,extending downward below the lowermost jacket module into the sea-bed,and extending upward out of the tops of the legs of the lowermost jacketmodule and into legs of a second jacket module located atop thelowermost jacket module such that the foundation piles extend into thelegs of the second jacket module by a height less than the height of thejacket modules; and follower piles for installation within the legs ofthe second jacket module located atop the foundation piles and extendingupward out of the tops of the legs of the second jacket module.

Thus, the kit comprises modules and piles, the length and size of thepiles being such that they can be installed as required in the structureof the first aspect. The jacket modules and/or piles may have featuresas discussed above in relation to the first aspect, and they may bearranged for installation as described above. The kit may also includefurther features as described above such as at least one additionaljacket module and additional follower piles, an upper structure and soon.

The J-tube concept described above is considered to be novel andinventive in its own right and hence, view from a further aspect, theinvention provides a J-tube system for passage of cables and the likefrom the sea-bed to the top of an offshore structure, the J-tubecomprising an angled section including a lower generally horizontal partfor receiving a cable running generally horizontally along the floor, abend that turns the cable through about a right angle, an uppergenerally vertical part so that the cable exits the angled sectionrunning generally vertically, and an open non-enclosed cable runextending from the top of the generally vertical part toward the top ofthe offshore structure. With this J-tube the cabling is hence passedfrom the generally vertical exit portion of the angled section to thetop of the structure without being enclosed in a tube. As noted above,omitting the vertical tubing makes the construction and installation ofthe jacket simpler, and also makes it easier to install cables throughthe J-tube. This aspect extends to an offshore structure or foundationpart of an offshore structure incorporating the J-tube. The foundationpart may be a jacket type foundation of the type described above. TheJ-tube system may include cable restraints spaced apart up the height ofthe modular structure, such as rings mounted on legs of the modules.Preferably, the J-tube is located adjacent to a leg of the foundationpart and may be clamped to the leg. In a preferred embodiment, theJ-tube can be rotated relative to the offshore structure or foundationpart in order to direct the lower generally horizontal part of theangled section in any desired direction. In a particularly preferredembodiment the J-tube arrangement comprises two J-tubes mounted adjacentto two opposite legs of the lowermost jacket module.

Whilst sea level and the sea-bed are referred to above it will beunderstood that the structures described above may also be used in anysuitable body of water, such as seas, oceans, estuaries, inland lakesand reservoirs. Hence, any reference to sea level should be understoodto mean a datum water level for the desired location of the structure inany body of water. Similarly, any reference to the sea-bed should beunderstood to refer to the bottom of the body of water.

As used herein the terms top and bottom and other terms relating torelative vertical location such as upper and lower are intended toreference the normal installed orientation of parts of the jacket andoffshore structure. Thus, for example, the top of a pile is the partthat is uppermost when it is installed, and the bottom of a pile is theopposite end of the pile. Naturally the parts of the structure may bemanufactured and transported in other orientations.

Certain preferred embodiments of the invention will now be described byway of example only and with reference to the accompanying drawings inwhich:

FIG. 1 is a partially exploded isometric view of a jacket and windturbine, with some ancillary parts omitted for clarity;

FIG. 2 is a side elevation of the jacket;

FIG. 3 is a plan view of the jacket with the transition piece omitted;

FIGS. 4 to 10 show the installation of the jacket on the sea-bed;

FIG. 11 is an illustration of the detail of the connection between pileswithin the legs of the jacket, with exaggerated diameter;

FIG. 12 is an elevation of a part of the installed jacket with gangwaywith the gangway retracted;

FIG. 13 is a similar elevation to FIG. 12, with the gangway deployed;and

FIG. 14 shows the detail of the connection between the lower portion ofthe gangway and a leg of the jacket.

The preferred embodiment is described in the context of a jacket for awind turbine, as shown in FIG. 1. The wind turbine and tower can be ofgenerally conventional construction and hence are not described in anyfurther detail. The tower is mounted on a transition piece 2 that isinstalled atop the jacket 4. The jacket 4 comprises three jacket modules6 secured to the sea-bed by piles 8. FIG. 1 shows the extent of thepiles beneath the jacket into the sea-bed. In this preferred embodimentthe jacket modules 6 are 12 m in height and width, with a generallycubic shape. The piles 8 may extend by around 30 m into the sea-bed, theactual depth being decided depending on soil conditions and on theexpected loading on the wind turbine. The jacket 4 is sized so as toallow for one of the three modules to protrude above the reference waterlevel.

FIGS. 2 and 3 show the jacket 4 in greater detail. Each, jacket module 6is a cube shape with four hollow legs 10 along the vertical edges, whichare joined on each vertical outer surface of the cube by a trussstructure formed by two cross-beams 12 in an X shape. The preferredembodiment shown in the drawings does not include any other cross beams.However, if required for structural strength then horizontal cross-beamscan be added above and/or below the X-shaped beams 12. As can be seenclearly in the plan view of FIG. 3 the jacket modules 6 do not includeany beams across the middle of the structure either inside the cubeshape or at the horizontal outer surfaces of the cube shape. Thepreferred embodiment uses tubular hollow beams. In the context of jacketdesigned to support an offshore wind turbine the hollow beams for this12 m cube have a diameter of 1.2 m for the hollow legs 10 and a diameterof 0.6 m for the cross-beams 12.

The lowermost jacket module 6 includes mud mats to support the jacketmodule on the sea-bed. The mud mats may be adapted according to soilconditions in a conventional fashion. The jacket 4 also includes J-tubes(not shown) for passage of cables and the like from the sea-bed to thetop of the jacket 4. The J-tubes (not shown) are mounted to the base oflegs 10 of the lowermost jacket module 6 and can be rotated. Each J-tubeincludes an angled section with a lower horizontal part that receives acable running generally horizontally along the floor, a bend that turnsthe cable through a right angle, and an upper vertical part so that thecable exits the angled section running vertically upward toward the topof the jacket. Unlike a conventional J-tube the J-tubes in thispreferred embodiment do not include a vertical section of tubing runningfrom the angled section to the top of the jacket. Instead, the cablingis passed from the vertical exit portion of the angled section to thetop of the structure without being enclosed in a tube. Rings mounted onlegs 10 of the modules 6 keep the cables in place during wave and tidalwater movements.

After installation of the jacket 4 the J-tube closest to the incomingcabling is rotated in order to direct the lower horizontal part of theangled section in the direction of the cabling. The cabling can then beinstalled through the J-tube and up to the top of the structure via therings.

The transition piece 2 comprises a central circular structure forsupporting a turbine tower and four triangular beams 14 extendingoutwardly away from the central structure and tapering toward outerconnecting parts at the end of each beam 14, which connect to the topsof piles extending upwardly from the uppermost jacket module 6. Thebeams 14 have a box beam construction.

Further details of the structure of the jacket 4 will be apparent fromthe following discussion of installation of the jacket 4, with referenceto FIGS. 4 to 10. The three jacket modules 6, transition piece 2 andpiles 8 are conveyed to the installation site by barge 16 and the first,lowermost, jacket module 6 a is lowered to the desired position on thesea-bed by crane. FIG. 4 shows a barge 16 with second and third jacketmodules 6 b, 6 b and the transition piece 2 on deck, and the firstjacket module 6 a located on the sea-bed. Four foundation piles 8 a areinserted into the hollow legs 10 of the first jacket module 6 a and asshown in FIG. 5 these are driven through the legs 10 and into thesea-bed. The pile driving is done partly in air and partly with thehammer submerged. As noted above, the extent of the foundation piles 8 ainto the sea-bed might be about 30 m. The upper end of the foundationpile 8 a is left protruding from the tops of the legs 10 of the firstjacket module 6 a.

For this example using 12 m high jacket modules 6 the exposed length ofthe pile is at least 4 m. Two of the foundation piles 8 a are exposed by4 m. The other two piles are left with a greater exposed height in orderto provide guide piles to facilitate simple alignment of the next jacketmodule. In this preferred embodiment a first guide pile extends 5.5 mabove the tops of the legs 10 and a second guide pile extends 4.5 mabove the tops of the legs.

As an alternative to driving the foundation piles 8 a into the sea-bedwith a hammer the piles could be installed in pre-drilled holes andsecured in place by grout or the like.

When the foundation piles 8 a are in place the second jacket module 6 bis lowered onto the exposed upper ends of the foundation piles 8 a. Thisstep is shown in FIG. 6. The second jacket module 6 b is first alignedwith the first guide pile, which is the foundation pile that extends outof the lowermost jacket module 6 a by the greatest height. The secondjacket module is landed on this first guide pile, and this provides afirst fixed point. Then, when the first guide pile is engaged with thesecond jacket module 6 b, then the jacket module can be rotate to alignwith the second guide pile. Landing the second jacket module on thesecond guide pile fixes the jacket module rigidly in place, preventingfurther rotation. When it is slid down the first and second guide pilesit should be correctly aligned with the remaining two foundation piles 8a, allowing it to be easily located on all four piles.

When the second jacket module 6 b is in place follower piles 8 b areinstalled in the legs 10 of the second jacket module 6 b. The followerpiles 8 b have a length that is the same as the height of the jacketmodules 6, and hence they protrude out of the tops of the legs of thesecond jacket module 6 b by the same length that the foundation piles 8a protrude from the lowermost jacket module 6 a. This means that thefollower piles 8 a will include first and second guide piles with extraheight corresponding to the extra height of the guide piles in thefoundation piles 8 a.

The third jacket module 6 c and a further set of follower piles 8 b arethen installed atop the second module 6 b in a similar fashion.

When all the jacket modules 6 and piles 8 have been installed grout ispumped into the space between the piles 8 and the jacket legs 10. Thegrout is pumped upward from the base of the jacket using conventionalgrout injection techniques and a grouting vessel 17, which is shownschematically in FIG. 8. The grout forms the connection between thepiles 8 and the jacket legs 10 and ensures that the jacket columns cancarry vertical tensile loads.

The exposed pile ends at the top of the partially constructed jacket arecut to make them level with each other, and to provide an exposed lengthsuitable for supporting the transition piece 2, which is installed onthe exposed pile ends as shown in FIG. 9. The transition piece 2 ispre-fabricated with an access way 18, which is not shown in FIG. 9 butis described below with reference to FIGS. 12, 13 and 14. Before thetransition piece 2 is installed it is rotated so that the access way 18is on the leeward side of the jacket 4. Since the transition piece 2 andthe other parts of the jacket 4 have a rotationally symmetricalstructure it can be installed in any rotational orientation relative tothe jacket modules 6.

With the transition piece 2 in the correct orientation the fourconnecting parts of the transition piece 2, which take the form ofcylindrical sockets with similar dimensions to the hollow legs 10 of thejacket modules 6, are placed on the top ends of the piles 8, and thengrout is injected to secure the transition piece 2 in place. The jacket4 is then complete, as shown in FIG. 10. The transition piece 2 mayoptionally be installed with the turbine tower already in place, thetower and transition piece 2 having been assembled together onshore.

Further details of the inter-module and inter-pile connections will nowbe described with reference to FIG. 11. FIG. 11 shows a cross-section ofthe jacket leg and piles at a join between two modules, with thediameter of the leg and piles exaggerated compared to the overlaplength. As explained above, the length of overlap would be 4 m in thisembodiment.

The hollow legs 10 of the jacket modules 6 include internal lugs 20protruding from the wall of the leg and acting to locate the piles 8 inthe centre of the hollow legs 10 and also to maintain a set minimumspacing between the piles 8 and the walls of the hollow legs 10. Inaddition, the legs 10 include a connecting ring 22, which in this caseis at the bottoms of the legs 10 of the upper module 6. The connectingring 22 has an inner diameter generally larger than the outer diameterof the legs and includes a tapered part that acts to guide the two legsinto alignment and to give a tight, wedged fit of one leg onto theother. An O-ring seal is also included, to ensure that the connectingring 22 seals the joint between the two legs 10 and contains the grout23 that is later pumped into the space between the piles 8 and the wallsof the hollow legs 10.

The two piles 8 are designed to interlock using a plug and socket typearrangement. The lower pile 8 has a male joint 24 consisting of atruncated cone protruding upward from a flat, with a shoulder about thebase of the cone. The shoulder can be used for impacting the hammer whena foundation pile 8 a is driven into the sea-bed. The upper pile 8 has afemale joint 26 of complementary shape, with a frustoconical volume forreceiving the truncated cone of the male joint 24. Advantageously, thecone shape provides a tight fit that prevents undesirable ingress ofgrout into the join between two piles 8. The foundation pile 8 a has themale joint 24 at its upper end. The follower piles 8 b are provided witha female joint 26 at one end and a male joint 24 at the other end.

FIGS. 12 and 13 show further detail of the constructed jacket and towerand ancillary parts that are installed on the transition piece 2. Theseancillary parts include a deck area 28 on the transition piece thatjoins to a gangway 30, and a crane 31. The crane 31 is used to loadparts and equipment onto the deck area 28 from a vessel 33 below. Thedeck area 28 also joins to a stairway up into the tower that providesaccess into the tower from beneath the base of the tower. Advantageouslythis avoids the need to cut a doorway into the tower, which means thatthe tower is not weakened by holes in its structure.

As noted above, the deck area 28 and gangway 30 are prefabricated on thetransition piece 2 and during installation of the transition piece 2,with or without the tower, it is rotated to place the gangway 30 on theleeward side of the jacket. The final stage of the installation processis to connect the gangway 30 to a leg 10 of the upper jacket module 6.Prefabrication of these ancillary parts with the transition piece 2means that complicated construction and assembly occurs on land ratherthan at the offshore installation site.

The gangway 30 provides access to the deck area 28 and turbine towerfrom a vessel. It is an articulated stairway with upper and lower partsin the form of flights of stairs 32, 34 that join the deck area 28 to anaccess platform 36 at the base of the stairway. The upper flight 34 isconnected to the deck area 28 at one end and extends away from the deckarea 28 to an intermediate platform 38. At the intermediate platform 38the direction of the stairway reverses and the intermediate platform 38connects to the lower flight 34, which runs back toward the deck area28, ending at the access platform 36, which in this embodiment islocated generally beneath the starting point for the upper flight 32,where it joins the deck area 28.

The upper and lower flights 32, 34 are rotatably connected to each othervia the intermediate platform 38 and the upper flight 32 is rotatablyconnected to the deck area 28. The rotating connections allow thegangway 30 to be lowered toward water level in order to place the accessplatform 36 at deck level for a vessel 33 below. When the gangway 30 isnot in use the access platform 36 can be retracted and raised to astowed configuration with the upper and lower flights 32, 34 foldingtogether. FIG. 12 shows the raised/stowed configuration of the gangway30 and FIG. 13 shows the extended/in use configuration.

A constant tension winch 40 on the deck area 28 is attached to theaccess platform 36 via cables and supports the vertical load that arisesfrom the weight of the gangway 30. The winch 40 is used to raise andlower the gangway 30. When a vessel 33 is present and the gangway 30 islowered the access platform 36 sits on the deck of the vessel. As thevessel 33 rises and falls due to water movement the constant tensionwinch 40 will raise and lower the gangway 30 to match the movement ofthe vessel 33. As a result the access platform 36 will not move up anddown relative to the vessel 33, which will make it easier to use thegangway 30.

An important feature of the gangway 30 is the connection of the base ofthe stairway to the leg 10 of the uppermost jacket module 6. This isshown in more detail in FIG. 14, which includes a close up view of thebase of the stairway shown in the deployed configuration with a vesselpresent. A collar 42 around the leg 10 guides the vertical movement ofthe access platform 36 and prevents it from moving horizontally. Thecollar 42 also provides a reaction force to support horizontal loadsarising from the weight of the gangway 30. It will be understood thatsince the stairway extends sideways from its mounting point at the deckarea 28 then a moment is created about the mounting point to the deckarea 28. The collar 42 provides a horizontal reaction against thismoment in order to support the offset gangway 30. The collar 42 includesguide wheels 44 so that it can move smoothly up and down the leg 10. Theconnection of the upper flight 34 to the deck area 28 also bears ahorizontal load, which is equivalent to the horizontal reaction at thecollar 42, but in the opposite direction.

The collar 42 is connected to the access platform 36 by a shaft 46. Alsoconnected to the collar 42 is a bumper 48. The bumper 48 is designed toreceive the prow of the vessel 33 and to hence keep the vessel 33 fixedin place horizontally. The vessel 33 can use a small amount of thrust tokeep the prow of the vessel 33 pressed against the bumper 48. The bumper48 will move up and down with the vessel 33 since, as with the accessplatform 36, it is supported by the constant tension winch 40.

1. A jacket for an offshore structure, the jacket comprising: aplurality of jacket Modules stacked one above the other; each of thejacket modules having multiple hollow legs; and a plurality of pilespassing through the hollow legs for securing the jacket to the sea-bed;wherein the piles include: foundation piles passing through legs of alowermost jacket module, with a lower part of each foundation pileextending downward below the lowermost jacket module into the sea-bed,and an upper part of the each foundation pile extending upward out ofthe tops of the legs of the lowermost jacket module and into legs of asecond jacket module located atop the lowermost jacket module such thatthe upper parts of the foundation piles extend into the legs of thesecond jacket module by a height less than the height of the jacketmodules; and follower piles within the legs of the second jacket modulelocated atop the upper parts of the foundation piles and extendingupward out of the tops of the legs of the second jacket module.
 2. Ajacket as claimed in claim 1, wherein the piles are arranged to extendupwardly out of the legs of the jacket module by different amounts, suchthat there is a first guide pile that extends upwardly by a greaterlength than the other piles, and a second guide pile that extendsupwardly by a length less than the first guide pile but greater than theremaining piles.
 3. A jacket as claimed in claim 1, comprising: a thirdjacket module atop the second jacket module, with the follower piles inthe legs of the second jacket module being a first set of follower pilesand extending upward into the legs of the third jacket module; and asecond set of follower piles within the legs of the third jacket modulelocated atop the first set of follower piles and extending upward out ofthe tops of the legs of the third jacket module.
 4. A jacket as claimedin any of claims 1, 2, or 3, wherein the piles are secured within thelegs of the jacket modules by grout.
 5. A jacket as claimed in anypreceding claim, comprising a transition piece for mounting a tower,such as a wind turbine tower, the transition piece being mounted on theexposed parts of the uppermost follower piles above the uppermost jacketmodule.
 6. A jacket as claimed in claim 5, wherein the transition pieceis pre-fabricated onshore and comprises: a central structure forsupporting a tower or the like and outer connecting parts for couplingto the exposed parts of the uppermost follower piles.
 7. A jacket asclaimed in claim 6, wherein the outer connecting parts are hollow tubesplaced on the exposed parts of the uppermost follower piles, and groutis used to secure the transition piece to the jacket modules.
 8. Ajacket as claimed in any preceding claim, wherein the jacket modulesinclude connectors for providing a seal about the joint between adjacenthollow legs.
 9. A jacket as claimed in any preceding claim, wherein thehollow legs include internal lugs for locating the piles at the centreof the legs and spacing the piles away from the walls of the legs.
 10. Ajacket as claimed in any preceding claim, wherein the ends of the pilesare shaped for interlocking fit with the next follower pile.
 11. Ajacket as claimed in any preceding claim, wherein the upper ends of theupper parts of the foundation piles comprise a shoulder around aprotruding conical or frustoconical element.
 12. A jacket as claimed inclaim 11, wherein the follower piles include a female coupling at theirlower end to accommodate the protruding part of the upper ends of thepreceding piles.
 13. A jacket as claimed in claim 12, wherein the other,upper, end of the follower piles comprise a shoulder around a protrudingconical or frustoconical element.
 14. A jacket as claimed in anypreceding claim, wherein the jacket modules are designed with anarrangement of legs that is rotationally symmetrical about an axisextending parallel with the legs.
 15. A jacket as claimed in anypreceding claim, comprising a J-tube with an angled section comprising alower generally horizontal part for receiving a cable running generallyhorizontally along the floor, a bend that turns the cable through abouta right angle, and an upper generally vertical part so that the cableexits the angled section running generally vertically, wherein theJ-tube does not include a vertical tube extending up the entire heightof the jacket.
 16. A jacket as claimed in claim 15, wherein the J-tubeis located adjacent to a leg of the lowermost module and can be rotatedin order to direct the lower generally horizontal part of the angledsection in any desired direction.
 17. A jacket as claimed in claim 15 or16, comprising two J-tubes mounted adjacent to two opposite legs of thelowermost jacket module.
 18. An offshore wind turbine comprising ajacket as claimed in any preceding claim.
 19. A method of installationof a jacket for an offshore structure, the jacket comprising: aplurality of jacket modules, each of the jacket modules having multiplehollow legs; and a plurality of piles; wherein the method comprises:locating a first, lowermost, jacket module on the sea-bed; installingfoundation piles through the legs of this lowermost jacket module suchthat lower parts of the foundation piles extend downward below thelowermost jacket module into the sea-bed and upper parts of thefoundation piles extend upward out of the tops of the legs of thelowermost jacket module by a height less than the height of the jacketmodules; locating a second jacket module atop the lowermost jacketmodule such that the upper parts of the foundation piles extend into thelegs of the second jacket module; and installing follower piles withinthe legs of the second jacket module such that the follower piles arelocated atop the upper parts of the foundation piles and extend upwardout of the tops of the legs of the second jacket module.
 20. A method asclaimed in claim 19, the jacket being as claimed in any of claims 1 to17.
 21. A method as claimed in claim 19 or 20, wherein the piles areinstalled such that they extend upwardly into the legs of the nextjacket module by different amounts.
 22. A method as claimed in claim 21,wherein the jacket comprises a first guide pile that extends upwardlyinto the legs of the next jacket module by a greater length than theother piles and a second guide pile that extends upwardly into the legsof the next jacket module by a length less than the first guide pile butgreater than the remaining piles, and the method comprises locating asubsequent jacket module by landing a first leg of the next jacketmodule on the first guide pile, aligning a second leg of the jacketmodule with the second guide pile and landing the second leg on thesecond guide pile so that the remaining legs and piles are aligned. 23.A method as claimed in any of claims 19 to 22, comprising grouting aboutthe piles inside the hollow legs in order to thereby secure the pileswithin the legs, wherein the grout is pumped into the legs around thepiles after assembly of all the jacket modules and piles is completed.24. A kit of parts for a jacket, the kit of parts comprising: aplurality of jacket modules arranged to be stacked one above the other,each of the jacket modules having multiple hollow legs; and a pluralityof piles arranged for passing through the hollow legs for securing thejacket to the sea-bed; wherein the piles include: foundation piles forpassing through legs of a lowermost jacket module, the foundation pilesincluding lower parts for extending downward below the lowermost jacketmodule into the sea-bed and upper parts for extending upward out of thetops of the legs of the lowermost jacket module and into legs of asecond jacket module located atop the lowermost jacket module such thatthe upper parts of the foundation piles extend into the legs of thesecond jacket module by a height less than the height of the jacketmodules; and follower piles for installation within the legs of thesecond jacket module located atop the upper parts of the foundationpiles and extending upward out of the tops of the legs of the secondjacket module.
 25. A kit of parts as claimed in claim 24, wherein thejacket modules and/or piles are as claimed in any of claims 1 to 17.